Polarization-maintaining (PM) fibers typically utilize a pair of stress rods, disposed longitudinally along opposing sides of the core region (symmetrical configuration) to create stress-induced birefringence within an optical signal propagating along the core region of the optical fiber, splitting the signal into orthogonally polarized modes. The stress rods are formed of a material exhibiting a high thermal expansion such that during the process of drawing an optical fiber from a preform, the rods create a stress state that is “frozen” within the final form of the fiber. The diameter (D) of each stress rod, as well as its displacement from the center of the core region (R1), determine the strength of the birefringence that may be achieved. Larger diameter rods and/or closer proximity of the rods to the center of the core region are preferred designs that create a state of higher birefringence (i.e., greater splitting between the orthogonally polarized modes).
PM fibers have successfully been used for many years, primarily in situations where standard single-mode fiber is employed; that is, fibers having core regions no greater than about 10 μm, with surrounding cladding layers sufficiently large enough to allow for optimum placement of relatively large diameter stress rods.
In situations requiring high power outputs, large mode area fibers may be employed, where these fibers are known to have core region diameters in excess of 40-50 μm. Most installations utilizing these high power optical fibers still require that the fiber is able to be coiled (reducing its ‘footprint’ at a particular location) and, therefore, a maximum fiber outer diameter is typically no greater than about 1 mm or so. Given these constraints, the ability to configure a high power PM fiber where the stress rods are separated from the large-sized core in the same relative relationship as used for conventional fibers is not a realistic option.
Additionally, as the core size increases, more modes are allowed to propagate, giving rise to unwanted mode coupling (including mode coupling of the fundamental mode to higher-order modes, as well as coupling among various higher-order modes). Inasmuch as birefringence splits the degeneracy of these modes along the “fast” and “slow” axes, the density of modes within a polarization-maintaining arrangement is an order of two larger (i.e., higher mode density) than conventional, non-PM large mode area fibers. It has been expected by those skilled in the art that this high density of states will prohibit operation with desired levels of modal purity and polarization extinction necessary for laser or amplifier operations. Moreover, this high mode density has been expected to be exacerbated by fiber bending, as a result of introduced asymmetry into the relationship between the stress rods and the core region.
Furthermore, besides adding the desired birefringence (typically on the order of >10−4) between the optical modes and maintaining a polarization extinction ratio (PER) that exceeds 10 dB over a typical fiber length of 1 meter, a useful PM, large-mode-area fiber is expected to maintain other characteristic properties such as a stable spatial distribution with well-separated propagating constants among the various guided modes.
These concerns, among others, would have one skilled in the art conclude that it is unlikely that a polarization-maintaining large mode area fiber useful for laser or amplifier applications can be achieved as a commercial product offering.